In one class of vehicles, such as double drum compactors telehandlers, some consumer sized trucks and the like, a compound steering system allows for steering via the front wheels or drum and/or the rear wheels or drum. The forward steering subsystem and the aft steering subsystem operate within angular limits defined by various mechanical linkages and stops known in the art. For example, the rotation axis of the front wheels may be able to be reoriented within mechanical limits between plus and minus 25°, and the aft steering system may have similar or different angular limits. These compound steering systems typically enhance vehicle maneuverability capabilities in a way well known in the art. For instance, in the case of a telehandler, the vehicle can maneuver in relatively confined spaces not realistically possible with a vehicle not having a compound steering system. In another example, a double drum compactor might have the ability to operate in an offset mode in which the forward and aft drums move over offset or overlapping paths when the compactor is moving forward.
In the past, there was typically a direct mechanical linkage between the operator steering input device (e.g. steering wheel) and the actuators that serve to reorient the wheels or drum relative to the vehicle chassis. In more recent years, that direct mechanical linkage has yielded to electronically controlled steering actuators that respond to electronic control signals in a well known manner. For instance, movement of the operator's steering input device sends a signal to an electronic control module that is interpreted, and then an appropriate control signal is sent to the electronic actuators operably coupled to the wheels or drums. Typically, the steering actuators might be electro-hydraulic steering actuators of a type known in the art.
Because of the lack of a mechanical linkage between the operator steering input device and the steering actuators, the art teaches the application of resistance to moving the operator steering input device in order to provide a more familiar feel to the operator and for other reasons known in the art. For instance, co-owned U.S. Pat. No. 6,389,343 to Hefner et al. teaches a steering resistance device for a vehicle with electronically controlled steering that creates artificial resistance on the operator steering input device via a variable viscosity fluid that surrounds the steering shaft. In one example, the variable viscosity fluid is either an electro-rheological or a magneto-rheological fluid whose viscosity related sheer properties can be varied by varying the strength of either an electric or magnetic field through the fluid, respectively. Hefner et al. also teaches the concept of increasing the artificial resistance to the operator input device adjacent the mechanical limits by increasing that resistance so as to alert the operator that the vehicle is at its turning limits, and further turning of the operator steering input device would produce no additional turning capability. Hefner et al., however, fails to contemplate compound steering systems and the unique circumstances associated with the same.
In one potential area of concern, when a double drum compactor is operating in an offset mode in which the forward and aft drums move over offset pathways, maintaining a predetermined offset distance while turning can be problematic. In some instances, the electronic control module is programmed to distribute turning commands between the forward and aft drums in order to maintain a predetermined offset. However, when the mechanical limit of one of those turning subsystems is reached, the predetermined offset can not be maintained. When one of the two steering subsystems in a compound steering system reaches its mechanical limit, the phenomenon is commonly referred to as an oversteer condition. If the operator is unaware that they have reached an oversteering condition when operating in an offset mode, the desired drum offset can be compromised, as the offset typically shrinks when operating in an oversteer condition. Thus, maintaining a predetermined offset when turning can be difficult for an operator to accomplish through observation techniques alone.
The present disclosure is directed to one or more of the problems set forth above.